Stop telling yourself, "I'm starting on Monday."
Start now, right now, this very second. Take it one decision at a time. Don't beat yourself up over bad decisions. Get back up, recover and move forward
Every day is a reset, a fresh opportunity to do better. Take advantage of it. Get a plan, prioritize things that really mean something to you. Deprioritize what doesn't. Set a schedule, write it down. Stick with it. As you slowly build this habit of resetting each day, you'll see things start to change, good habits start to build and bad habits fading away. It will take time, you'll have set backs, but it will get easier.
One more time - better not perfect.